Optical systems that focus incoming light while preventing distortions are known in the art. Refraction of light occurs when the light beam passes from one medium to another, where each medium has a different refractive index, thereby causing the light beam to bend or change direction at the interface between the two media. Since the refractive index of a glass lens is sensitive to the wavelength of the light, the lens will refract light made up of at least two different wavelengths, so that each wavelength is refracted by a different amount and comes to a focus at a different focal point, resulting in a phenomenon known as chromatic aberration. Chromatic aberration is a constant source of disturbance when imaging a scene, since the position of the focal point is different for each wavelength that images the scene, resulting in colors (color “fringes”) at the edges between high contrast regions of the image. For example, forward looking infrared (FLIR) systems, which image a scene with wavelengths in the near infrared (NIR) and mid-wavelength infrared (MWIR) ranges, where the wavelengths of MWIR can be substantially four times the wavelengths of NIR, are prone to such chromatic aberrations.
There are two types of chromatic aberration. One type, known as “longitudinal chromatic aberration”, is described as the inability of a lens to focus different wavelengths in the same focal plane (i.e., the different wavelengths are focused at different distances from the lens, along the optical axis). When focusing parallel light, only longitudinal chromatic aberration takes place. Obliquely incident (non-parallel to the optical axis) light exhibits another type of chromatic aberration, known as “transverse (or lateral) chromatic aberration”. In this case, the foci are displaced in a sideward direction in a plane perpendicular to the optical axis. Various techniques that reduce or correct chromatic aberration are known in the art, such as the use of achromatic lenses and apochromatic lenses.
Reference is now made to FIGS. 1A and 1B, which are schematic illustrations of longitudinal and lateral chromatic aberrations, respectively, as known in the art. In FIG. 1A, parallel light from a scene 50, made up of two wavelengths (λ1, λ2), passes through optics 60, and comes to a focus at two distinct focal lengths along optical axis Z. The shorter wavelength λ1 comes to a focus at a first focal length Flength1, whereas longer wavelength range λ2 comes to a focus at a second focal length flength2, thus exhibiting the phenomenon of longitudinal chromatic aberration. In FIG. 1B, oblique light from scene 50, made up of two wavelengths (λ1, λ2), passes through optics 60, and comes to a focus at two distinct focal plane widths. Shorter wavelength range λ1 is focused at a first focal plane width fwidth1, whereas longer wavelength range λ2 is focused at a second focal plane width fwidth2, thus exhibiting the phenomenon of lateral chromatic aberration.
In a publication entitled “Use of diffractive elements to improve IR optical systems”, SPIE, Vol. 4820, 2003, pp. 744, Nevo et al. disclose the replacement of conventional optical arrangements by diffractive ones in order to improve the performance of infrared (IR) optical systems. An optical system disclosed by Neva et al. consists of an objective composed of zinc selenide (ZnSe) and germanium (Ge) lenses, where one side of the Ge lens surface has a diffractive pattern etched onto it. The objective lens is achromatic and passively athermalized, as known in the art. An alternative optical system is proposed which includes an objective made of indium antimonide (InSb) as a focal plane array (FPA) sensor at MWIR spectral range. The optics includes zinc sulfide (ZnS), calcium fluoride (CaF2), silicon (Si) and Ge lenses, such that the requirement is to image a 1.06 micron laser spot on the InSb sensor. A new design, composed of one front ZnS lens and five additional ZnSe lenses, enables the transmittance of both the MWIR and the 1.06 micron spectral bands. Two diffractive surfaces are added to correct for chromatic aberration at the MWIR wavelength. A third diffractive surface focuses the 1.06 micron spot in the same plane as the MWIR image.